The present invention relates to a method and apparatus for adjusting anodes in an electrolytic cell. More particularly this invention relates to a method and apparatus for generating voltage and current signals for use in adjusting the anode-cathode spacing in mercury cells and also for automatically raising the anode with respect to the cathode to prevent overcurrent and short circuits.
Electrolytic mercury cells have been used commercially in the production of chlorine and caustic by the electrolysis of brine for many years. In general these cells employ a metal cell container which slopes slightly downwardly from one end to the other, and which utilizes a cathode comprised of a moving stream of mercury on the bottom of the cell. A stream of brine flows on the top of the mercury cathode in the cell container. Anodes, fabricated either from graphite or metal, are secured to the top of the cell container and positioned in the brine above the mercury cathode.
When a voltage is applied across the cell, current flows from the anode through the brine electrolyte to the cathode and causes electrolysis of the brine and the formation of gaseous chloride, which is removed from the cell, purified and stored. Elemental sodium, another product of the electrolysis forms an amalgam with the mercury cathode and is removed from the cell and processed to form a caustic solution. Regenerated mercury from the amalgam is recycled for use as a cell cathode.
A plurality of electrolytic mercury cells are normally electrically connected in an electrical series circuit. Each cell usually has a plurality of anode buses which are electrically connected to the cathode side of an adjacent cell. In some instances the anodes of each cell may be divided into sets with each set of anodes of a given cell being separately adjustable to raise and lower the anodes of a given set with respect to the mercury cathode. Each anode set may be provided with one or a plurality of anode buses. With cells of this design, a motor is associated with each anode set to raise and lower the anodes of the set.
According to another cell design, all the anodes of the cell are raised and lowered simultaneously. In this case, a motor is associated with the entire group of anodes to raise and lower the anodes.
Whatever the design of the cell, the control of the inter-electrode distance between the anode and cathode is economically important. The inter-electrode distance should be as small as possible to reduce the wasteful consumption of energy.
In addition to the problem of maintaining the optimum anode-cathode spacing in mercury cells, the problem of preventing short circuiting due to contact between the mercury cathode and anode is also of importance. Such short circuits may be caused by breakage of a graphite anode, by loosening of anode support posts, by changes in the thickness of mercury due to faulty flow control, or other causes which allow the anode to contact the flowing mercury cathode. The resulting short circuit causes an excessive flow of current in the anode and in the anode bus serving that anode, along with anode damage, overheating of the anode leads, loss in production of chlorine, excessive hydrogen in the chlorine, and other problems. In addition, with metallic anodes, a short circuit damages the active coating and the support structure which cannot be economically tolerated.
Numerous techniques have been developed for adjusting the anode-cathode gap in electrolytic cells. See, for example, U.S. Pat. Nos. 3,574,073, which issued Aug. 6, 1971; 3,873,430, which issued Oct. 25, 1975; and 3,900,373, which issued Aug. 19, 1975. In general these patents describe a method of adjusting the anodes by transmitting current and voltage signals to a computer or to some type of visual readout or both. The operator, during manual control of the anode sets, can raise or lower a particular anode set through a motor control system until the desired operating condition is reached. Alternatively, these systems may utilize a digital computer which can perform various operations and actuate the motor control circuit to raise or lower the anode sets until the desired condition is reached.
In addition, such patents as the aforementioned U.S. Pat. No. 3,574,073, as well as U.S. Pat. No. 3,844,913, issued Oct. 29, 1974, disclose systems for the automatic raising of the anodes or anode sets of a given cell upon the sensing of an overcurrent or short circuit condition.